Source: Starfinder Core Rulebook p.158
You have trained to make your unarmed attacks lethal and strike with kicks, head-butts, and similar attacks.
Benefit: Your unarmed attack damage increases to 1d6 at 4th level, 2d6 at 8th level, 3d6 at 12th level, 5d6 at 15th level, and 7d6 at 20th level. You threaten squares within your natural reach with your unarmed strikes even when you do not have a hand free for an unarmed strike. If you are immobilized, entangled, or unable to use both legs (or whatever appendages you have in place of legs, where appropriate), you lose the ability to make unarmed strikes without your hands. When making an unarmed strike without your hands, you can’t use such attacks for combat maneuvers or similar abilities—only to deal damage.
Normal: You don’t threaten any squares with unarmed attacks, and you must have a hand free to make an unarmed attack.